hmm those who are facing problem with lags after updated to 4.1jb.
please do following.
1) use custom rom ( you have to root the phone and have to compromise on inbuilt feature such as track on samsung website but same can be neglected after using free avast antivirus)

Anonymous, 18 Aug 2013Had nothing but trouble with the phone since I've had it. Sent it back three times to be fixed... moredelete the antivirus and install it once in a while just to check for viruses . since i deleted it i play most games without lag and my battery lasts longer . also the phone doesnt hang so often as it did when i had an antivirus.

Hi friends those who facing hanging and lagg problem on jb 4.1.1 my suggestion is to root u r phone and install cynogenmod 10.1 21/06 built. its a great rom but with some bugs like no wifi tether, omx but one can live without that. But you can feel the smoothness of s advance after that and u will love this phone again no hang at all, phone become blazing fast, u can multitask more app without freeing up u r ram
In future all bugs will be solved devs are working on that follow this thread u will get everything what u want
forum.xda-developers.com/forumdisplay.php?f=1952­